1. Field of the Invention
The present invention relates to an ignition timing control method and apparatus in a system for sensing knock in an internal combustion engine and retarding the ignition timing, which apparatus prevents the ignition timing from being substantially retarded by abnormality of the retarding system.
2. Description of the Prior Art
In the spark ignition type engine, it is usually desirable to set the ignition timing to an MBT (Minimum spark advance for Best Torque), but as engine load increases, an ignition timing limit for knocking is exceeded when the ignition timing is set to MBT and the knocking occurs.
Furthermore, since the ignition timing limit for knocking changes with engine temperature, fuel octane value and the amount of deposits in the combustion chamber, together with the reasons described above, it is necessary to set the ignition timing to a considerably retarded point. Consequently, engine torque is not maximized.
Heretofore, as an approach to solve the above problem, U.S. Pat. No. 4,002,155 discloses a method in which a vibration of the engine is sensed by a vibration sensor and the ignition timing is set to the MBT by an ignition timing control means in accordance with the conditions of the engine (such as r.p.m. and suction vacuum), and when the knocking of the engine is detected by a knocking detection circuit in response to the output of the vibration sensor, the ignition timing is retarded to effectively utilize the torque of the engine.
However, in the prior method, if the knocking detection circuit erroneously determines the knocking due to a failure occuring between the vibration sensor and the knocking detection circuit, or if the knocking detection circuit erroneously detects the knocking because electric noise is added to the output of the vibration sensor to produce an output different from a true vibration output, the ignition timing is unduly retarded to make the operation of the engine difficult.